1. Field of the Invention
This invention relates to a method of producing aromatic aldehydes and more particularly to an improved method of carrying out the Sommelet reaction.
2. Description of the Prior Art
The reaction of monohalomethylaromatics with urotropin (hexamethylenetetramine), giving aromatic aldehydes, is an old-established reaction referred to as the Sommelet reaction.
In this, monohalomethylaromatics are reacted with urotropin (hexamethylenetetramine), giving the corresponding arylalkylurotropinium halide as an intermediate which undergoes rearrangement to the salt of a Schiff base; at the same time, methyleneimine is formed by reaction of formaldehyde and ammonia, the ammonia resulting from the decomposition of the urotropin. Ultimately, the aldehyde and methylamine are formed from these two molecules by hydride shift. A comprehensive review has been given by S. J. Angyal, Org. Reactions 8 (1954), 197 et seq. It emerges from this article and also from disclosures elsewhere that pure monohalomethylaromatics, for example benzyl chloride, p-xylyl chloride and the like, are required as starting materials if the reaction is to be carried out as a one-vessel reaction.
In general, the reaction has hitherto been carried out in solvents, such as alcohol, aqueous alcohol or glacial acetic acid.
Where an unpurified starting material was used, earlier authors have recommended that the intermediate, namely the arylalkylurotropinium halide, should be isolated and only then used to carry out the Sommelet reaction.
Using these methods, the maximum yield achieved is, according to Angyal, loc. cit., about 70% of theory.